The present invention relates to a drop-on-demand printer of the type in which an agglomeration of particles is created and then ejected, by electrostatic means, onto a printing substrate. More particularly, the invention relates to such a printer having a row of ink ejection locations for ejecting plural ink droplets, such as described in our WO-A-93-11866.
Such printers may be manufactured with very small spacings between adjacent ink ejection locations, in which case, it is desirable to reduce electrostatic cross-talk between adjacent locations or channels. This can be achieved by incorporating guard channels between pairs of ejection channels. Such printers are usually operated by means of a bias voltage applied continuously to the ejection locations through appropriat ejection electrodes and, when ejection is required, applying suitable pulse voltages to the ejection electrodes. The bias voltage may also be continuously applied to the guard channels. However, when the ejection electrodes associated with two or more adjacent ejection locations are pulsed continuously, a high field is created between the ejection locations and the intervening guard channels and fluid may be forced from the ejecting n locations to the guard channels and from there may be ejected onto the substrate. It is desirable therefore to reduce the possibility of such erroneous ejection.
According to the present invention therefore there is provided a drop-on-demand printer having a row of ink ejection locations for ejecting plural ink droplets, each ejection location having an associated ejection electrode to which a voltage is applied for causing electrostatic ejection of the droplets from the respective ejection location; a guard channel disposed between adjacent ejection locations, each guard channel having an electrod disposed therein; and control means for applying a voltage to said guard channel electrodes, said applied voltage being the average of the voltages applied in operation over a given time to the adjacent ejection location electrodes.
A second aspect of the invention includes a drop-on-demand printer having a row of ink ejection locations for ejecting plural ink droplets, each ejection location having an associated ejection electrode to which a voltage is applied for causing electrostatic ejection of the droplets from the respective ejection location; a guard channel disposed between adjacent ejection locations, each guard channel having an electrode disposed therein; and control means for applying a voltage to said guard channel electrodes, said applied voltage being the average of the voltages applied in operation over a given time to the ejection location electrodes.
The control means also preferably applies a bias voltage to the guard channel electrodes with which the average voltage is summed. Also preferably, each guard channel electrode is connected to a bias voltage through a capacitance. Similarly, each guard channel electrode may be connected to the adjacent ejection location electrodes through resistances of equal value.
The invention also includes a method of operating a drop-on-d mand inkjet printer having a row of ink ejection locations for ejecting plural ink droplets, each ejection location having an associated ejection electrode for causing electrostatic ejection of the droplets from the respective ejection location, and a guard channel disposed between adjacent ejection locations and having an electrode disposed therein, the method comprising applying a voltage to said guard channel electrodes, said applied voltage being the average of the voltages applied in operation over a given time to the adjacent ejection location electrodes.
Further, the invention includes a method of operating a drop-on-demand ink jet printer having a row of ink ejection locations for ejecting plural ink droplets, each ejection location having an associated ejection electrode for causing electrostatic ejection of the droplets from the respective ejection location, and a guard channel disposed between adjacent ejection locations and having an electrode disposed therein, the method comprising applying a voltage to said guard channel electrodes, said applied voltage being the average of the voltages applied in operation over a given time to the ejection location electrodes.
The invention also includes a drop-on-demand printer having a row of ink ejection locations for ejecting plural ink droplets, each ejection location having an associated ejection electrode to which a voltage is applied for causing electrostatic ejection of the droplets from the respective ejection location; a plurality of guard channels disposed between adjacent ejection locations, each guard channel having an electrode disposed therein; and control means for applying a voltage to at least some of said guard channel electrodes.